The need for very high data transmission, on the order of 1 Gbit/s over short distances, on the order of a few meters, is one of the drivers behind the development of transmission equipment for use in the 60 GHz frequency band. Because there is about 7 GHz available around 60 GHz, this means that very large data rates can be supported still having quite relaxed requirements on spectrum efficiency.
Typical application where this kind of equipment can be envisioned is for high speed modes of WLAN, for wireless High Definition Multimedia Interface (HDMI), etc.
Although there is plenty of bandwidth available, there is an inherent drawback by operating at 60 GHz, namely that the propagation attenuation is increased significantly as the attenuation is proportional to the carrier frequency squared. This is the reason why only short distances can be supported with reasonable transmission power. However, the large propagation attenuation also has an advantage in that the interference from other transmitters using the same band will quickly decay to below the thermal noise floor so that it will not have any noticeable impact.
As one of the envisioned applications of very high data transmission is for consumer electronics, and often as cable replacement, it can be expected that it is desired that the transmitter and receiver are as low-complex as possible in order to allow for low cost implementation as well as low power consumption.
As the data rate increases, the bit duration, Tb, will decrease correspondingly, which means that the delay spread due to multi-path propagation will be more of an issue and this would normally require channel equalization. A rule of thumb is that in an equalizer is needed in case the rms delay spread of the channel exceeds 10% of Tb.
For a data rate of 1 Gb/s, Tb is 1 ns, so that the delay spread must not exceed 0.1 ns according to the above rule of thumb, in order to avoid the need for an equalizer in the receiver.
A delay spread this small does in practice correspond to that only one single transmission path exists between the transmitter and the receiver. Because of the large propagation attenuation mentioned above, the delay spread for a channel measured at 60 GHz will be considerably smaller than if the channel is measured at for instance 2.4 GHz. Although the delay spread in fact has been found to be relatively small, several components in the multi-path profile are typically experienced. To further reduce the delay spread, and sometimes also to effectively reduce the multi-path channel to a single path, multiple antennas at the transmitter and the receiver may be used. The idea is essentially to use beam-forming to further suppress all but the strongest component in the multi-path channel.
Seong-Gu Lee et. al. “Performance Analysis of Beamforming Techniques in Ad-hoc Communication between Moving Vehicles”, in Proceedings of Asia-Pacific Conference on Communications, 2007 found that the rms delay spread can significantly be reduced by using beam-forming techniques. However, in particular in the context of consumer products, it remains desirable to provide a low-complex system that can operate at high data rates while at the same time reducing the power consumption.